The invention relates to a method for minimizing the effect of interference in a radio system which comprises at least one subscriber terminal and a base transceiver station which receives signals from said subscriber terminal, and in which method power control requests are transmitted to said subscriber terminal.
In practice, transmitters used in radio networks are non-linear, and a part of a signal ends up outside the frequency band allocated to the transmission. When a signal ends up outside the frequency band allocated to it, it causes interference which is called adjacent channel interference (ACI).
The non-linearity of a transmitter is closely related to the properties of the amplifier of the transmitter. In general, it can be said that linear amplifiers cause minor interference to adjacent frequency bands, but the performance of the amplifiers is relatively low. Instead, non-linear amplifiers cause more interference in relation to the non-linear amplifiers. However, non-linear amplifiers have the advantage that they provide a better performance than the linear amplifiers.
Adjacent channel interference causes problems especially in wide-band systems, such as the WCDMA (wide-band code division multiple access) system. For instance, the WDCMA specification includes a maximum value for interference caused by the adjacent channel.
FIG. 1 shows a curve which describes the increase in noise in a CDMA system in relation to the system load. The load is proportional to the number of subscriber terminals in the system. The curve is obtained by the following formula:                               r          =                      10            ⁢                          log              ⁢                              (                                  1                                      1                    -                    η                                                  )                                                    ,                            (        1        )            
wherein
r is the amount of noise exceeding thermal noise,
xcex7 indicates the degree of loading of the system.
The load of the CDMA system as a function of the users can be calculated as follows:                               η          =                                                    (                                                      n                    F                                    -                  1                                )                            ⁢              d                                                      G                p                            /                              (                                                      E                    b                                    /                                      N                    0                                                  )                                                    ,                            (        2        )            
wherein
Gp is the processing gain,
d indicates the voice activity,
F is the reuse coefficient of the frequency,
n indicates the number of users,
Eb/No indicates the ratio of the energy of the received bit to the thermal noise density.
The coefficient F obtains the value 1 when this concerns a separate cell. The coefficient F can obtain the value 0.9, for instance, when this concerns a typical micro cell. When this concerns a typical macro cell, the coefficient F obtains the value 0.67, for instance.
When the formulas (1) and (2) are combined, the following formula is obtained for the amount of noise exceeding thermal noise:                     r        =                  10          ⁢                      log            (                          1                              1                -                                                                            (                                                                        (                                                      n                            /                            F                                                    )                                                -                        1                                            )                                        ⁢                    d                                                                              G                      p                                        /                                          (                                                                        E                          p                                                /                                                  N                          o                                                                    )                                                                                            )                                              (        3        )            
FIG. 2 shows curves 2a, 2b and 2c defined for different cell types, obtained by the formula (3), when typical values are inserted in the variables of the formula. The curve 2a shows a load curve of a macro cell, the curve 2b shows a load curve of a micro cell, and the curve 2c shows a load curve of a separate cell. FIG. 2 shows that in a micro cell a higher noise is allowed than in a macro cell.
FIGS. 1 and 2 show that when the degree of loading- is between 0.5 and 0.8, the noise increase is in the range of 3 to 6 dB. When the degree of loading is in the above-mentioned range, a small increase in the degree of loading only causes a small increase in the amount of noise. Because the curve rises steeply, it is easy to see that if the degree of loading is higher than the high value of the above-mentioned range, even a small increase in the degree of loading causes a high increase in the amount of noise. The higher the load in the system, the higher the number of subscriber terminals that need to increase their transmission power. However, an increase in transmission power further increases the amount of noise in the network, and the system may become unstable.
In radio networks, an admission control algorithm is used to ensure that for a call there are sufficient resources which allow a sufficiently good signal-to-interference (SIR) ratio and bit rate for the call signal. The admission control algorithm is, for instance, applied when a subscriber terminal begins to establish a connection in a new cell. The admission control algorithm can also be applied during handover.
The admission control algorithm is applied separately in the uplink and downlink directions, especially when the traffic volumes of the different directions differ a great deal from each other. When the algorithm is activated, call set-up can be prevented.
Radio networks also use a load control algorithm which is used to try and maintain the network resources in a predefined range of use. The admission control and load control algorithms contain a parameter indicating noise increase. The parameter and, at the same time, the algorithm can, for instance, be activated when the amount of noise exceeds a predefined noise limit.
Applying the load control algorithm requires constant processing in which interference is monitored. The algorithm is used in defining the load factor. If a predefined load factor value is exceeded, the network reduces the bit rate of the users whose service contract allows the reduction of bit rate. In addition, the network delays transmission of the users who have no requirements with respect to delay. The network can also interrupt low-priority calls.
In some situations, the network can, in principle, drop all its calls. Calls have to be dropped when the noise level remains too high even after all available means have been used to reduce the noise level.
When the network is underloaded, the load control algorithm increases bit rates of those users who are able to process higher bit rates. Increasing or reducing bit rates can be done in order of priority, for instance.
Let us now examine the situation in FIG. 3, which clearly shows the problem which arises as a result of using an adjacent channel. In the figure, a subscriber terminal MS1 is connected to a base transceiver station BTS1. The connection uses a frequency F3 in the uplink direction (MS1 xe2x86x92BTS1). The terminal MS1 is, however, close to another base transceiver station BTS2 which receives on a frequency F4 from a terminal MS2 of its own.
If the frequencies F3 and F4 are adjacent frequency bands in the frequency range, the base transceiver station BTS2 will experience the transmission of the terminal MS1 as adjacent channel interference, because the selectivity of the receiver in the base transceiver station BTS2 is not ideal. Problems would arise even though the receiver did operate selectively, because the signal of the adjacent channel also spreads to the reception band of a selectively operating receiver. The problem situation caused by the interference is especially difficult when BTS1 and BTS2 are base transceiver stations of different network operators, for instance, since then the terminal MS1 cannot make a handover to the base transceiver station BTS2.
It is thus an object of the invention to provide a method for minimizing the effect of interference and a radio system so as to reduce the above-mentioned problems. This is achieved by a method of the type as claimed in the preamble, which is characterized by separately measuring the transmission frequency of power control requests transmitted to a subscriber terminal, and when the transmission frequency of the power control requests transmitted to the subscriber terminal exceeds a predefined transmission frequency of power control requests, by altering the attenuation of signals arrived at a base transceiver station at least in the case of the signals which are transmitted from the above-mentioned subscriber terminal to the base transceiver station.
The object is also achieved by a method of the type as claimed in the preamble, which is characterized in that for executing the method, there are two method step groups, of which at least one is executed in the method; in the first method step group: the transmission frequency of power control requests transmitted to each subscriber terminal is measured separately, and when the transmission frequency of power control requests transmitted to the subscriber terminal exceeds a predefined transmission frequency of power control requests, the attenuation of signals arrived at a base transceiver station is altered at least in the case of the signals which are transmitted from the above-mentioned subscriber terminal to the base transceiver station; in the second method step group: the size of interfering signals arrived at the reception band of the base transceiver station is defined, the activation threshold of the algorithms controlling the operation of the radio system is changed when the measuring result obtained from measuring the interfering signals exceeds a pre-set limit value.
Further, the object is achieved by a method of the type as claimed in the preamble, which is characterized by separately measuring the transmission frequency of power control requests transmitted to each subscriber terminal, increasing the attenuation of signals received by a base transceiver station until one subscriber terminal can no longer receive the service provided by the base transceiver station, and thereafter reducing the attenuation of the signals arriving at the base transceiver station from the subscriber terminal, the signals being those arriving at the base transceiver station from the subscriber terminal, the transmission frequency of power boost requests transmitted to which exceeds a predefined transmission frequency of power boost requests.
The invention also relates to a radio system which comprises at least one subscriber terminal and a base transceiver station which is adapted to receive signals from said subscriber terminal and which base transceiver station is adapted to transmit power control requests to said subscriber terminal.
The system of the invention is characterized in that the radio system comprises a measuring means which is adapted to measure the transmission frequency of power control requests transmitted to the subscriber terminal, and an attenuation means which is adapted to alter the attenuation of signals arriving at the reception band of the base transceiver station and transmitted by the subscriber terminal, the transmission frequency of power control requests transmitted to which has exceeded a predefined transmission frequency of power control requests.
The system of the invention is also characterized in that the radio system has two groups of means available to it, the radio system comprising at least one of them at each time; and the first group of means comprises: a measuring means which is adapted to measure the transmission frequency of power control requests transmitted by the base transceiver station to each subscriber terminal, and an attenuation means which is adapted to alter the attenuation of signals arriving at the reception band of the base transceiver station and transmitted by the subscriber terminal, the transmission frequency of power control requests transmitted to which has exceeded a predefined transmission frequency of power control requests; and the second group of means comprises: a measuring means which is adapted to define the size of interfering signals arrived at the reception band of the base transceiver station, a control means which is adapted to change the activation threshold of the algorithms used in the radio system, when the result obtained from the above-mentioned definition of interfering signals exceeds a pre-set limit value.
The system of the invention is further characterized in that the radio system comprises a measuring means which is adapted to measure the transmission frequency of power control requests transmitted to the subscriber terminal, and an attenuation means which is adapted to increase the attenuation of the signals received by the base transceiver station until one subscriber terminal can no longer receive the service provided by the base transceiver station, and the attenuation means is adapted to reduce the attenuation of the signals transmitted from the subscriber terminal to the base transceiver station, the signals being those arriving at the base transceiver station from the subscriber terminal, the transmission frequency of power boost requests transmitted to which by the base transceiver station has exceeded a predefined transmission frequency of power boost requests.
Preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on altering the operation point of a base transceiver station suffering from interference by taking the load of the base transceiver station into account. In addition, the sensitivity of the base transceiver station is improved, which station during a given time transmits power boost requests, the number of which exceeds a pre-set limit.
The method and radio system of the invention provide several advantages. The operation point of a base transceiver station suffering from interference is altered by taking into consideration especially the load caused by the adjacent channel to the base transceiver station, in which case the effect of the interference on the base transceiver station can be reduced. The effect of the interference can be reduced in a relatively simple manner. The reduction requires that the receiver of the base transceiver station has an attenuation means which, when altering its attenuation value, takes into consideration the number of power control requests transmitted by the base transceiver station during a given time.
The attenuation of the attenuation means is altered adaptively, which helps prevent coverage area losses of the base transceiver station. The attenuation of the receiver is kept as high as possible. The attenuation is reduced to some extent when it is detected that a subscriber terminal is no longer able to increase its transmission power. When the sensitivity of the base transceiver station improves, the subscriber terminal can reduce its transmission power and thus causes less interference.
In adjusting the attenuation, the transmission frequency of transmitted power reduction requests is also taken into account as follows. When the number of power reduction requests transmitted to the subscriber terminal exceeds a predefined limit value, the receiver of the base transceiver station increases the attenuation it uses.
The effect of interference is also reduced by temporarily increasing the threshold level of activating the load control and admission control algorithms, when the size of the interference exceeds a pre-set limit. This way, the base transceiver station does not immediately need to disconnect connections, for instance, but the interfering element is provided some extra time to stop interfering. The method and system of the invention are especially suited for reducing the effect of interference caused by an adjacent channel.